Powders of metals, ceramics, plastics, etc. having particle diameters of several .mu.m or larger which are coated with a certain metal by electroless plating, chemical replacement plating based on ionization series, CVD, etc. are used for catalysts, ornaments, powder metallurgy, dispersed reinforcing particles for composite materials, materials which provide electromagnetic shielding materials with electric conductivity, etc. Powders used for such purposes should preferably be as small as possible because the specific surface area of a powder increases with decreasing particle size and thus powders of smaller particle sizes have better sintering properties, reactivity, etc. Therefore, development of techniques by which fine particles of smaller diameter can be coated with a wider range of materials is strongly desired.
JP-A 56-130469(1981) discloses a process for preparing powders coated with a noble metal which have the same appearance as said noble metal and can be used for ornaments, said process comprising coating a fine powder of glass, a ceramic or a plastic which has a diameter of not smaller than 50 .mu.m with a noble metal by sputtering as the powder is being cascaded on an inclined surface.
However, it is believed to be extremely difficult to firmly and uniformly coat fine particles smaller than 10 .mu.m with a metal or a ceramic by the conventional processes mentioned above or by PVD process such as sputtering, ion plating, etc, and successful coating of this type has not been reported. The difficulty arises because fine powders with particle diameter smaller than 10 .mu.m exhibit strong aggregation and thus easily form secondary particles, and may adsorb moisture and various gases.
Under the circumstances, the inventor disclosed in JP-A 62-250172(1987), a process for coating ultrafine powder of a metal, a ceramic or a plastic whose primary particles have an average particle diameter of 100 .ANG. to 1 .mu.m with a metal, a ceramic or a plastic, said process comprising:
(a) dispersing the particles of said powder into primary particles by fluid-jet-milling in an inert atmosphere and carrying the particles by an inert gas; PA1 (b) heating the dispersed ultrafine powder in a heating zone of an inert atmosphere under a reduced pressure; PA1 (c) dropping the powder in a sputtering chamber provided with vertical targets so as to fall in the direction parallel with said targets and thus coating said powder with a metal, a ceramic or a plastic by sputtering; and PA1 (d) repeating the procedures of the above (a), (b) and (c). PA1 (1) It takes a long time to uniformly coat each particle of a fine powder having a particle size of 0.1 .mu.m to 10 .mu.m by sputtering since the powder has a vast surface area. It was further found that the sputtering time is determined by the free falling velocity of the falling particles in the sputtering chamber (vacuum) while the sputtering (coating) proceeds at almost the same speed as in the atmosphere. Because of this, it is often difficult to uniformly coat heavy metal powders particle by particle. PA1 (2) In the case of light powders, it was found that powder particles diffuse during free falling in the sputtering chamber and some of the powders tend to adhere to the targets and hinder the normal sputtering. PA1 (a) dispersing particles of said fine powder into primary particles by fluid jet mill treatment in an inert atmosphere; PA1 (b) heating the dispersed fine powder in an inert atmosphere under reduced pressure; PA1 (c) placing the heated powder in a rotary barrel sputtering chamber, forming a fluidized bed by rotating the barrel at a low speed, and coating said fluidized powder by sputtering as the barrel is being rotated. PA1 (1) a sputtering chamber comprising an evacuable rotary barrel supported by two separate rotation shafts, wherein a sputtering source or sources are supported by the first rotation shaft, and the second rotation shaft functions as a conduit for introducing fine powder to be coated and taking out coated fine powder; PA1 (2) a fluid jet mill means communicating with the sputtering chamber via a conduit provided with a valve and the second rotation shaft which functions as a conduit for taking out coated powder; PA1 (3) an evacuable heat-treating vessel communicating with said fluid jet mill means via a conduit provided with a valve; and PA1 (4) a powder-introducing conduit provided with a valve which communicates said heat-treating vessel and said second rotation shaft.
However, it was discovered that the process involves the following problems:
This invention is intended to solve the above problems and provide a sputtering process and apparatus for coating the surface of primary particles of a diameter of 0.1 .mu.m to 10 .mu.m firmly and uniformly with a metal or a ceramic.